Vent field photograph. taken at the East Pacific Rise during the Overalpping Sreading Center cruise of 2007. Courtesy of E. Klein.

1. PROGRAM OVERVIEW & HIGHLIGHTS OF RIDGE 2OOO RESEARCH

"Seafloor ecosystems are inextricably linked to, and perhaps an inevitable consequence of, the flow of energy and material from Earth's deep mantle, through the oceanic crust to the deep ocean."
Ridge 2000 Science Plan

Ridge 2000 (R2K) was founded with the ambitious goal of understanding oceanic spreading center systems 'from mantle to microbe'. Key objectives include: i) characterizing the processes that control the structure of newly-formed oceanic crust, so that the reasons and scale for natural variability/similarity of typical oceanic plates, a majority of the planet, are understood; ii) documenting the extent to which microbial communities mediate exchange of material and energy between the solid Earth and the overlying oceans; iii) determining how chemosynthetic ecosystems develop and are supported via hydrothermal circulation of seawater within new oceanic crust, thus investigating the related question of how life may have evolved on early Earth. To address these questions, Ridge 2000 focuses on specific geographic sites, that are compared and contrasted to evaluate generality and global significance of ridge processes. The practical vision was to bring together scientists who do not traditionally undertake collaborative research, to conduct coordinated field experiments. This process was envisioned as the key to achieving the integration of results that is required for a whole-system understanding of mid-ocean ridges.

The main theme of Ridge 2000 is Integrated Studies and within this theme the Program also emphasizes Time Critical Studies, which take advantage of current geologic activity on the spreading axis to investigate and document short-term evolutions of the system. Consistent with NSF's emphasis on a Merit Review criterion addressing broader impacts, the Program also supports a strong education and outreach component that aims to bring the excitement of Ridge 2000 research to public, K-12, and undergraduate audiences.

A unique aspect of the Program has been the detailed coordination of interdisciplinary field programs. The Program has facilitated the collection of data sets that are now allowing the relationships between linked geophysical, geological, geochemical, and biological processes to be recognized for the first time. This was not a trivial task, and the Ridge 2000 community has worked through a variety of complexities to develop approaches that now set the standard for interdisciplinary deep-sea research that has been emulated by other nations.

Because of the strongly inter-disciplinary nature of R2K inquiry, the Program is designed to enable advances at the interface between traditional research avenues. The Program facilitates the activities of a diverse community of researchers in 5 primary ways: 1) information gathering and dissemination by the R2K Office; 2) workshops for research planning and scientific synthesis; 3) Integrated Studies Site/Time Critical Studies experiment coordination; 4) education and outreach; and 5) database development and data archiving. Section 2 provides a summary of R2K research activities to date, with more detail on program operations available in Section 5. Section 3 looks ahead at expectations for the second phase of the research Program (2008-2012). Section 4 covers education and public outreach. Section 6 provides specific measures of Program impacts. The rest of Section 1 highlights the goals of the program and the research progress made during the first phase (2002-2007) of R2K.

1.1. Overarching Goals of the Program

R2K integrated studies aim to develop quantitative, whole-system models of oceanic spreading center processes, through coordinated field and laboratory/modeling studies. The field efforts are centered at a small number of specific geographic locations (Integrated Studies Sites- ISS) where multi-disciplinary experiments are co-located in time and space. This approach is designed to reveal the geological and petrological processes of seafloor spreading, heat and chemical exchange associated with hydrothermal circulation, the composition and activity of ecosystems, and the relationship among these processes that are supported in the basaltic crust and hydrothermal chimneys at each site and in the proximal ocean.

Time critical studies (TCS) aims to understand the nature, frequency, distribution, and geo-biological impacts of magmatic and tectonic events along oceanic spreading centers. To accomplish this goal, TCS facilitates rapid-response missions to observe the immediate biological, chemical, and geological consequences of transient events on the seafloor. Ecologically, vents are an intriguing system for study because of their transience. At some spreading ridges, this environmental variability occurs on time scales similar to species generation times. Capturing an eruption event, and the ecosystem's response to it provides a basis for broader understanding of how populations persist in highly variable/dynamic systems and the relative importance of various geological and geochemical processes in controlling the system(s).

The main aspects of the spreading center system under investigation can be outlined as follows. Partial melting of the mantle within the upwelling zone beneath an oceanic spreading center is a first step in the creation of new oceanic crust. Mantle temperature, composition (including volatiles), upwelling rates, and the plate spreading rate all influence the balance between tectonic and magmatic control of crustal structure along the spreading axis. The porosity and composition of the crust, and the depth and rate of magmatic intrusion or eruption, create the framework that determines the patterns/rates of seawater circulation, heat and chemical exchange, development of diffuse or localized venting, and colonization by subsurface microbial communities. Thermal and chemical gradients are probably greatest near the rapidly venting hydrothermal chimneys, though their influence extends into the diffuse flow in the surrounding environs. Both the chimneys and diffuse flows provide a unique biohabitat for megafauna and microbial species. Organisms indigenous to vents are the most thermotolerant life forms known, and some of the microorganisms are even capable of modulating "bio-remediating" the toxicity of this high-pressure, extreme chemical environment. Their activity allows larger invertebrates to thrive, attaining biomass densities comparable to rainforests. Fundamental research continues into every one of the components of the system, from mantle to microbe, as do efforts to determine the extent of interplay between the geological, biological, and chemical processes.

Within the framework of these interdisciplinary studies, R2K research programs are answering fundamental questions. Some of these questions arose early in the study of mid-ocean ridges, when hydrothermal circulation was still a hypothesis to explain 'missing' heat flow at the axis. Others were posed upon discovery of deep-sea hydrothermal vents and their fauna. Several of these initial questions have been honed to more specific queries as results of the preceding Ridge Inter-Disciplinary Global Experiments (RIDGE) program unfolded.

Representative questions that the R2K Program addresses include:

  • What aspects of mantle flow and melting determine crustal morphology and segmentation, and is there significant interplay between hydrothermal cooling and these characteristics?
  • In what ways do crustal structure and composition influence the intensity and composition of hydrothermal venting and the biological communities that populate a given system? Are there biochemical feedbacks that modulate hydrothermal circulation and crustal alteration rates?
  • Is the presence of molten magma in the crust required to drive hydrothermal venting, or is residual heat due to past intrusions sufficient, thus making tectonic controls more important for the evolution of hydrothermal systems and the biological communities they support?
  • How do the organisms survive in extreme conditions? How are populations maintained in transient habitat? What controls species abundance, distribution, diversity within vent communities?
  • What novel biochemical and physiological processes occur in the microbial and metazoan faunas?

The linkage between large scale, deep-Earth, inorganic processes and the smallest scale organic processes at the seafloor that is the essence of the R2K program is illustrated by a specific example (shown in the following figure)- tracking sulfur through the whole mantle, crust, hydrothermal, oceanographic, and ecological system.

Tracking Sulfur Mantle-to-Microbe figure

Images in Tracking Sulfur Mantle-to-Microbe figure are taken from the following publications:

1. Jull, Kelemen, & Sims, 2002 (Appendix 1).
2. Rubin & Sinton, 2007 (Appendix 5).
3. modified from original image by Gary Massoth.
4. Houghton, Shanks & Seyfried, 2004 (Appendix 1).
5. Glickson, Kelley, & Delaney, 2007 (Appendix 5).
6. McCollum & Seewald, 2007 (Appendix 1).
7. Rickard & Luther, 2007 (Appendix 1).
8. Shank et al. 2003 (Appendix 5).
9. Van Dover and Lutz, 2004 (Appendix 5).

Concept for figure by Bill Seyfried, layout and captions by Donna Blackman with input from George Luther, Tom McCollum and Jay Miller.

1.2. Discoveries and Key Developments

The R2K Program has already produced a number of discoveries and developments that are key to advancing understanding of spreading center systems and their associated biological communities. Highlights include:

  • Discovery of numerous new vents in Lau Basin. Many hydrothermal plumes were detected along the Lau Basin spreading centers, and associated vent fields at 5 of these sites were imaged. Before these 2004/2005 discoveries, the occurrence of venting had been predicted (e.g. Baker and German, 2004) but only a single vent field had been known in the basin. This was the first of several recent (R2K and other) nested studies that, together, indicate that vent fields are common along a variety of types of spreading centers.
  • Detection of unexpectedly high diversity in vent fluid chemistry. The diversity of hydrothermal fluid temperatures and chemistries is wider than was expected prior to R2K- host rock chemistry, phase separation effects, and magma degassing all appear to contribute to this variability.
  • Temporal variation in vent fluxes can be significant and changes are not always associated with an earthquake swarm or diking (Lilley et al., 2003, Kellogg et al., Fall AGU 2006).
  • Recognition of potential importance of anaerobic biological production. The potential for a significant contribution of anaerobic primary production (via the reductive TCA cycle), entirely disconnected from photosynthesis, has been recognized and this suggests the potential importance of alternative electron acceptors (e.g., NO3).
  • Advancements in understanding biology of Riftia pachyptila. Continuing physiological studies of the tubeworm Riftia pachyptila, make it arguably the most intensively investigated vent or deep-sea species, and demonstrates: 1) extremely high productivity potential by vestimentiferan tubeworms, comparable to the most productive ecosystems on Earth (e.g. kelp forests; Girguis et al., 2006); 2) hemoglobin binds sulfide by using zinc, as opposed to iron (Flores et al 2005); 3) two different carbon fixation pathways are potentially functional in the Riftia symbiont; (Markert et al., 2007) 4) evidence for horizontal transmission of symbionts, and continued environmental exchange in adults (Bright and Sorgo et al., 2005); 5) demonstration that symbionts are acquired after host metamorphosis and settling.
  • Refinement of upper thermal limit to life. R2K investigators have obtained results addressing the controversy over the upper thermal limit to animal life; Paralvinella sulfincola actively seeks out high temperatures, up to roughly 50°C (Girguis and Lee, 2006). The temperatures at which microbial populations can survive has also been shown to be higher than previously expected (Kashefi et al., 2005).
  • Removal of oceanic DOC by hydrothermal circulation. The magnitude of dissolved organic carbon in the deep-sea is similar to that of atmospheric CO2; high temperature hydrothermal circulation has been shown to remove the DOC component (Lang et al., 2006).
  • Discovery of new species. Several new microbial and macrofaunal species have been characterized including, but not limited to: EPR 62— an alpha protobacterium and EPR 70— a gamma proteobacterium (Vetriani, et al., unpublished); Strain T469 of DHVE2— thermoacidophilic archeon (Reysenbach et al. 2006); and a siboglinid polychaete— a tiny vestimentiferan tubeworm (Fisher et al., unpublished)
  • Deep-sea vehicle and sensor development. R2K has been a leader in recent technological advances for deep-sea research instrumentation. Sensor design (Section 6.4) represents direct scientific/engineering achievement. Project design (e.g. systematic, nested approaches or seafloor navigation at the meter scale) represent contributions to the general conduct of marine science.

1.3. Hypotheses Tested and Advances

First phase R2K research has provided tests of several prevailing hypotheses of spreading center processes. Many of these studies are still underway so results are not yet in hand. However, a number of hypotheses have already been confirmed or, in other cases, a shift in understanding has occurred as new findings clarified misconceptions and improved formulation of new hypotheses to be tested.

Examples of prior hypotheses that Ridge 2000 research has confirmed

  • The recurrence interval of magmatic activity at fast-spreading axes was hypothesized to be decadal in scale. Monitoring at the EPR ISS enabled capture of a 2005/2006 eruption(s) that provided completion of at least one form of volcanic cycle, with the previously documented 1991-1992 eruption(s) serving as the starting point (Cowen et al., 2007). Changes in fluid chemistry (Von Damm, 2004) and seismicity rates (Tolstoy et al., 2006) over ~2 yrs leading up to 2005 were used to predict that an eruption was imminent, and this was indeed the case.
  • Increased proximity of the Eastern Lau Spreading Center to the Tonga volcanic arc was hypothesized to influence the crustal structure, composition, hydrothermal circulation, and biological communities. This has been shown to be the case. Many components of the axial system show a threshold behavior, with rapid change over a short distance (biology in correspondence with crustal chemistry, for example), but other geochemical signatures (e.g. volatile contents of rock and fluid samples) show a steadier gradient along the ridge axis (Asimow and Langmuir, 2005; Goddard et al., Vancouver Comm. Meeting Poster, 2005; Langmuir et al., 2006; Proskurowski et al., Fall AGU 2007).

Examples of prior models that Ridge 2000 research has shown were incorrect

  • Some segments of the Juan de Fuca ridge (and by inference, other intermediate-rate spreading centers) were believed to lack an axial magma chamber; hydrothermal flow was thought to be driven by residual heat from past magma injection. Seismic imaging has shown this not to be the case- a melt lens is present along much of the spreading axis. (Canales et al., 2005)
  • Two-layer brine systems initially proposed to control vigorous hydrothermal venting at black smokers have been predicted to be unlikely. Modeling (Fontaine and Wilcock, 2006) suggests that phase separation will not lead to a stably-stratified brine layer.
  • Magma associated with spreading was inferred to be delivered mainly within the axial zone. However at the East Pacific Rise (EPR) ISS, petrologic (Sims et al., 2002), magnetic (Bowles et al., 2006), magneto-tellurics (Key and Constable, 2002), seismic imaging (Toomey et al, 2007), and seafloor mapping (Soule et al., 2005) all indicate that off-axis magmatism occurs. It is yet to be determined whether such activity is associated with significant off-axis hydrothermal activity, an instance of which was discovered by a RIDGE-funded study.
  • Magmatism in back-arc basins was inferred to reflect varying contributions of a single subduction component. In contrast, detailed sampling along the Lau spreading centers shows that multiple subduction components occur in close proximity and these co-mingle with background chemistry that varies somewhat along the mantle wedge defined by the basin.
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